Diamonds are one of the hardest substances known and they possess a number of desirable physical properties including hardness, chemical inertness, high thermal conductivity, and high electrical resistivity. Consequently, diamonds have become important for use in a variety of emerging technological devices, including semiconductors, heat sinks, abrasives, drill bits for use in drilling and in rock quarrying, and optical devices and lasers. Given the scarcity and expense of natural diamonds for such uses, there has been considerable interest in the development of synthetic diamonds. One process for producing synthetic diamonds and particularly diamond films is known as chemical vapor deposition (CVD). In the past several years, a variety of chemical vapor deposition techniques have been developed for producing high quality diamond films for use in optical devices, semi-conductors, and the like. For example, U.S. Pat. Nos. 5,114,745, of Jones, 5,270,077 of Knemeyer et al., and 5,523,121 of Anthony et al. disclose methods and techniques for producing diamond films.
One of the main problems associated with the production of diamonds and diamond films has been the control of the growth rate of the diamond material and control of roughness of the surface of the diamond. Many uses for diamond films require that the surfaces of these films be polished and substantially smooth, especially for heat management applications and for optical and tribological uses. Consequently, various methods have been developed for polishing the initially relatively rough surface of diamond films. Some of these methods primarily have focused on the use of various metals such as iron or nickel under conditions of high heat and pressure. For example, one of the principle techniques has been to apply molten iron or another liquid ferrous metal directly to the rough surface of a diamond film. Upon contact, the liquid iron molds itself to the contours, i.e., the peaks and troughs, of the rough surface of the diamond and the carbon in the diamond film diffuses into the iron to form iron carbide. Prolonged exposure to the molten iron causes the surface of the diamond film to diffuse away to a smooth or substantially smooth condition as the carbon on its surface is slowly converted to iron carbide. The problem with such a method is that it tends to dissolve away a significant amount of the diamond from both the peaks and valleys of the rough diamond surface, thus wasting a significant amount of diamond material.
This, in turn, leads to greater expense, and further requires significant time. In addition, it is very difficult to achieve a diamond surface with other than a flat configuration with this process.
Other smoothing techniques or methods include polishing the diamond film with a rotating metal polishing plate, typically formed from iron, nickel, and/or molybdenum, and at high temperatures and pressures. The polishing operation usually is carried out in a vacuum or within an inert atmosphere of hydrogen, nitrogen, helium, and/or argon gas in the polishing chamber. (See Tokura, Yoshikawa, Polishing of CVD Diamond Film; Applications of Diamond Films and Related Materials (1991).)
U.S. Pat. Nos. 5,403,619 and 5,468,326 of Cuomo et al illustrate a method of polishing a diamond film using an ionic conductor and the application of oxygen. Oxygen anions are produced by formation of vacancies in the superionic conductor and react with the diamond or carbon nitride of the diamond film to erode and thus smooth the peaks of the diamond film.
The problem with the latter two polishing methods generally has been the time and expense required to accomplish the polishing of the CVD and other synthetic diamond films. Both processes are carried out at relatively low rates on the order of 1 micrometer per hour for the Yoshikawa process and approximately 0.19 micrometers per half hour for the process of Cuomo et al. Thus, for diamonds with high surface roughnesses, i.e. 30-200 micrometers peak to valley, these processes can take up to several days to smooth and polish a diamond surface. These processes further require specialized polishing chambers and equipment that increase their cost. In addition, such methods are limited in that the difficulty of controlling the process tends to limit the number of diamond films that can be polished during each polishing operation.
In attempts to minimize the drawbacks of conventional polishing techniques, it has become possible through the development of improved chemical vapor deposition production techniques to create thicker diamonds and diamond films of higher optical and thermal quality and an initially smoother surface. Obviously, the cost of polishing a rough diamond decreases significantly as the initial smoothness of the surface of the diamond increases because less material must be removed to flatten the surface of the diamond to the desired smoothness. This is particularly beneficial for diamonds that are to be formed with non-planar surfaces that must be highly polished to a mirror-like smoothness. Accordingly, greater emphasis has been placed on the production of diamonds with initially smoother surfaces to reduce or even eliminate the need to polish the surfaces. For example, U.S. Pat. Nos. 5,114,745, 5,270,077 and 5,523,121 all relate to the formation of diamonds that are grown with substantially smooth surfaces to eliminate the need for polishing. The problems with such methods typically relate to the costs and time required for production of such diamonds and the consequent limits on production rates. In addition, it is difficult to produce diamonds with nonplanar surfaces, especially with nonplanar surfaces that are highly polished to mirror-like smoothness. Thus, the production of diamonds with surfaces that do not need polishing has not proven to be a complete solution.
Accordingly, it can be seen that a need exists for a method of polishing and smoothing diamonds rapidly, safely, with inexpensive materials and equipment, and without the requirement of increased ambient pressures. The process should be usable to polish large area planar or nonplanar diamond surfaces and be capable of polishing multiple diamonds or diamond films at the same time to increase production rate. It is to the provision of such a method that the present invention is primarily directed.